Drivers which allow an insufficient distance between their vehicle and the vehicle ahead of their vehicle may cause a traffic accident if the leading vehicle suddenly slows down. In the event of the driver of the leading vehicle responding to a situation with hard braking, the driver of the following vehicle may have insufficient time to notice and react to the illumination of the leading vehicle's brake lights, determine the required severity of braking, and decelerate sufficiently to avoid running into the rear of the leading vehicle. This error in judgment of a safe separation distance appears to increase at higher speeds, often resulting in catastrophic or even fatal accidents. The introduction of a center high mounted stop light (CHMSL) has greatly improved the rate at which the following driver notices the illumination of the leading vehicle's brake lights. However, this does not help reduce the driver's reaction times or vehicle braking rates.
Typical driver reaction times may vary between approximately 0.5 seconds and approximately 1.5 seconds, depending on how much a driver may be “driving ahead”, that is, monitoring traffic activity ahead of the leading vehicle. The vehicle's braking system also has a response time, and the degree of braking achieved depends on the weight of the vehicle, the speed of the vehicle, the design of the brake system, the state of wear or repair of the brake system, the design and degree of wear of the vehicle tires, the road surface, weather conditions and/or any other design parameters or conditions which may affect the stopping rate or deceleration of the vehicle. Many of these factors may also affect the ability of the driver of the following vehicle to control the vehicle path under severe braking conditions, which often results in an increase in the damage caused during this type of accident.
Many organizations concerned with road safety publish recommendations for safe separation distances between vehicles driving along a road. In some cases, these recommendations are made in meters, feet or car lengths at particular speeds quoted in kilometers per hour or miles per hour. In other cases, the driver is advised to note the distance traveled by his or her vehicle in a particular number of seconds as a separation guide. However, the driver's inability to retain this advice, relate the advice to a particular traffic condition at any time, and adhere to the advice is one of the causes of the type of accident described above.
A vehicle driver typically has two main concerns in this regard. First, whether the separation distance behind a leading vehicle is sufficient, and second, whether a following vehicle is too close to be able to react safely in the event of an emergency. Forward separation is the responsibility of the vehicle driver and is generally under his or her control. However, rearward separation is the responsibility of the driver of the following vehicle and is generally not under the control of the driver of the leading vehicle. However, it is not uncommon, if a driver is threatened by the closeness of a following vehicle, for the driver of the leading vehicle to tap the brake pedal in order to illuminate the vehicle brake lights, thus causing the following driver to brake and increase the separation distance between the vehicles.